Method and apparatus for producing fluid cavitation

ABSTRACT

A method and apparatus for producing fluid cavitation is provided. For a given channel cross-section of a pressure line, the flow is accelerated so as to reach a speed at which Re&gt;Re cr , where Re is the Reynolds number and Re cr  is the critical Reynolds number; the flow is then interrupted for a time less than half of the phase of the hydraulic shock. The device for cavitation of fluid flow is mounted in the channel of a pressure line and includes a cavitator which has the form of a working body placed in the casing and can move radially inside the line and, to a limited extent, along the axis of the line. The maximum cross-section surface of the working body in a plane perpendicular co the axis of the line is more than 0.8 of the passage of the line but not equal to it.

REFERENCE TO RELATED APPLICATIONS

Applicants claim priority under 35 U.S.C. §119 of Russian applicationNo. 99120729, filed Oct. 4, 1999. Applicants also claim priority under35 U.S.C. §120 of PCT/RU00/00392, filed Oct. 2, 2000. The internationalapplication under PCT article 21(2) was not published in English.

FIELD OF THE INVENTION

This invention pertains to hydrodynamics predominantly related to an oiland gas industry and also this invention can be used for other purposessuch as eliminating of microbiological objects, cleaning of surfacesfrom deposits, erosive breaking (pitting) of metals, promoting ofchemical reactions, dispersing of solid particles or high molecularcompounds into liquid, making emulsions of non-soluble compounds, and inother processes to improve effectiveness of internal mass transfer.

PRIOR ART

A method of cavitation of a liquid is known (I. S. Pearsall, Cavitation,Mills & Boon Ltd., London, 1972, pp. 9-16) which is referred to asvibrational method. It comprises an oscillating body (for example, amagnetostriction vibrator) that generates waves of pressure anddecompression in the ambient fluid. At certain magnitude of acceleration(oscillation frequencies) the pressure during the decompression phasereduces down to atmospheric thus providing a rupture of the fluidcontinuity and a cavitational cavity is formed which collapses duringthe counter phase.

The main shortcomings of such prior art method are the following:

1. The cavitation zone (i.e. zone of the fluid discontinuity) islocalized in the disturbance area adjacent to the oscillating body,though the pressure oscillations spread far remotely to the liquid;

2. The cavitation zone is stationary;

3. As the hydrostatic (external) pressure grows the fluid rupturebecomes impossible.

Another method of cavitation of a liquid known as hydrodynamical method(R. T. Knapp, J. W. Daily, F. G. Hammitt, Cavitation, McGraw Hill BookComp., N.-Y., 1970, pp. 13-35) comprises placing into a fluid flow of abarrier (for example, a body having a shape poorly followed by the flow)at the downstream part of which a zone of reduced pressure is formed. Atcertain critical speed of the fluid flow the pressure in this zonedecreases down to the atmospheric one resulting in generation of bubblesfilled by gas or vapor and, further then, a cavity. When the bubbles orcavities coming off the cavitator they pass into the higher pressurezone where they implode releasing some energy which can be usefullyapplied, for example, for cleaning of inner surface of a conduit from acorrosion layer or carbonate deposit.

This method of cavitation of a liquid is the most relevant by itsimplementation to the presently claimed one and therefore it isconsidered as a prior art prototype.

The main shortcomings inherent to the said prototype are the following:

1. The cavitation zone is formed, according to the cavitation number, atcertain magnitudes of the flow speed and ambient hydrostatic pressure;

2. The cavitation zone (cavity) is localized and formed along the flowedbody (cavitator) and is stationary;

3. It is impossible to rupture the fluid (produce a discontinuitycavity) at higher hydrostatic pressures, for example, the ones that aretypical for deep wells.

The devices are known to cavitate a fluid flow (U.S. Pat. No. 4262757, E21 B 7/18, 1981; “Oil and Gas J.”, 1977,31/X, v. 75, N 45, pp. 129-146)that are made in form of a barrier rigidly fixed in the direction of aflow (transverse bar, curved blade, cone directed counter flow,extensions of the duct into the flow, etc.). Such devices could beconsidered as analogs. Main shortcomings inherent to these devices areas follows:

1. According to the cavitation number

 σ=2(P−Pv)/ρV ² or σ=2(P+γz−Pv)/ρV ²

where P and Pv are, respectively, the pressure values in non-disturbedand disturbed flow; ρ—fluid gravity; z—depth (hydrostatic pressure);V—velocity of the non-disturbed flow respectively to the cavitator,γ=pg, where g is free falling acceleration.

It follows that too high fluid pumping rate is required to provide arupture of a flow continuity which rates are difficult to obtain,especially in deep wells or long pipelines;

2. It appears to be impossible to obtain cavitation due to such devicesat high hydrostatic pressure values, for example in deep wells.

The devices are also known, for example (J. W. Daily and D. F. Harleman,Fluid Dyamics, Addison-Wesley Ltd., Ontario, 1966, pp. 418-424)comprising a cavitator in the form of a ball rigidly fixed on a rodeplaced in the downstream part of the flow which device could beconsidered as a prior art prototype due to that it is the most close byits designing principles to the presently claimed ones.

Main shortcomings inherent to this prior art prototype are:

1. Ball closes less than 0.8 of the cross-section area of the conduitand is motionless, and therefore either very high fluid pumping rates orthe corresponding narrowing of the conduit (as it is usually employed inhydrodynamic setups to model the cavitation) is required to obtaincavitation and produce a cavitation cavity;

2. If a cavitation is obtained and the cavitation cavity is formed, suchcavity will not come off the cavitator since it is stationary, and as aresult, it is impossible to provide the effective action of cavitationon the surrounding bodies at a phase of imploding of the bubbles andcavities;

3. Applying of excessive external or internal pressure results indegeneration of cavitation (boiling of the liquid behind the cavitator),where just an underpressure zone will take place only.

SUMMARY OF THE INVENTION

The present method of cavitation of a flow of liquid appears to be thehydrodynamical one by its nature. The method is realized under thefollowing conditions: the flow in a given cross-section of the higherpressure delivery conduit is to be accelerated to a velocity at whichRe>Re_(cr), where Re—Reynolds number, Re_(cr)—critical Reynolds number,and then the flow is interrupted during a time less than duration of ahalf of semi-period of the liquid hammer. Due to such interruption, fullor partial, a rupture of the fluid flow is provided. The selection ofthe interrupt time less than semi-phase of a liquid hammer excludes theliquid hammer that is potentially harmful for the pressure part(manifold) of the conduit. To facilitate the fluid flow rupturing athigher hydrostatic pressure the nuclei of cavitation can be introducedinto the pumped fluid such as gas bubbles or dispersion of solidparticles or emulsion of an insoluble liquid.

The claimed device to cavitate the fluid flow in the delivery conduitcomprises a cavitator made in the form of a working body placed in thechannel of the conduit and said body has an opportunity to move in aradial direction of the conduit (casing) and is restricted to move alongthe axial direction of the conduit, and maximal area of cross section ofthe working body in a plane, perpendicular to axis of the conduit ismore than 0.8 of the cross-section of the conduit but not equal to it.

The claimed method of cavitation employs the kinetic energy of a fluidflow that, as it is known, is a function of a mass and velocity of amoving liquid. At a bigger length of a pipeline the force applied torupture a fluid can reach very high values thus enabling the solution ofa task to obtain cavitation at higher hydrostatic pressure. In wells of3000-5000 m by depth the hydrostatic pressure is equal to 300-500kg/sq.cm and more, and it is practically impossible to producecavitation under such conditions due to vibrations or just high pumpingrates. Similar conditions of high hydrostatic pressure can take place inthe on-land pipelines also.

The free turbulent vortexes nucleate the cavitation, i.e. create in theflowing liquid the local underpressure zones. These vortexes concentratethe gas dissolved in the liquid thus promoting the development ofcavitation. Injection into a fluid of gas bubbles, solid particles oremulsions of an insoluble liquid boost the said effect.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated in the FIGS. 1-12 where the devices arepresented with different geometry of a working body to implement theclaimed method of cavitation of a fluid flow at higher hydrostaticpressures, and specifically in the deep wells.

FIGS. 1-4 show the working body made in the form of a ball.

FIGS. 5-8 show the working body made in the form of a cone.

FIGS. 9-12 show the working body made in the form of a cylinder.

FIG. 13 shows an example of realization of the invention in a well.

FIG. 14 shows the oscillograms of working pressure pulsations at upperand lower parts of the cavitator recorded at a laboratory setup.

DETAILED DESCRIPTION

A working body 2 is placed inside the conduit 1 in a fluid flowing inthe direction as indicated by arrows. The limiter of axial motion 3 ismade in a form of a support.

In the FIGS. 1, 5 and 9 the working body 2 is placed on a support 3, inthe FIGS. 2, 6 and 10 the working body 2 is connected to the support 3via a flexible bound 4, in the FIGS. 3, 7 and 11 the working body 2 isplaced under the support 3 and connected to it via a spring 5, in theFIGS. 4, 8 and 12 the working body is placed over the support 3 andconnected to it via a spring 5.

When employing the proposed method (FIG. 13) to create cavitation voids10 in a fluid flowing inside a casing pipe 7 having perforation holes 8for hydraulic connection to a rock 9 bearing the fluids (oil, gas,water) a column of tubes 6 is used as a fluid delivery conduit 1 to theworking body (cavitator) 2.

The FIG. 14 testifies the fact of discontinuity of the fluid flow behindthe cavitator and forming of cavities. In this FIG. 14 an oscilloscoperecordings of pressure pulsations are shown as obtained in the setupcomprising two inertialess pressure sensors placed, respectively, in apoint over the ball working body (upper sensor) and under it (lowersensor). The pumping rate of fluid through the delivery pipe conduit was13.6 liters per sec, pressure at a distance of more than 3 m counterflow from the working body 2 was P=40 kg/sq.cm. In said experiments asteel ball of 76 mm by diameter was used as a working body placed freeon a rigid support 3 providing the closing ratio of the conduit channelof 0.85. The discontinuity of the flow (formation of the cavitationcavities) is proved by the record of the lower sensor which showscrossing by signal of a zero pressure line.

For cavitator suspended on a support 3 via spring 5 or placed on asupport 3 via spring 5, an opportunity is provided for a more completeseparation of the cavitation cavities from the working body 2 due tolongitudinal oscillations of the last. The frequency of the longitudialoscillations could be derived from an equation S=f*d/V, whereS—Strouchal number, f—frequency rate of the cavities separation,V—velocity of the flow, d—diameter of the cavitator. The Strouchalnumber (dimensionless criteria) is derived as a ratio of diameter of thecavitator to diameter of the conduit channel. Force of the spring shallbe selected from a condition that the natural frequency of oscillationsof cavitator to be close to the frequency of the frequency rate of theturbulent disturbance of the flow.

EXAMPLE

Employing one of the devices shown in the FIG. 1 as a fluid flowinterrupter one can produce hydro fracturing of porous rock in theborehole.

Standard hydrofracturing of the rock in boreholes is produced viasetting of a packer in the annulus above the perforation zone andinjecting of a liquid into the under-packer zone at a pressure higherthan the rock pressure and the rock rupture strength. As a result ofthat the liquid penetrating into incipient cracks in the rock broadenand deepen them in radial direction. To prevent closing of the cracksthey are fixed by sand or other propant.

The practices are also known to produce the rock fracture by shock waves(for example, USSR Certificate of Authorship N 973805, E 21 B 43/26,1982 or other). Shock wave having high peak magnitude is capable torupture a rock, provided the wave front is directed by normal to theborehole wall acting through the perforation holes in a casing. Howeverthe effect of a shock wave lasts only few milliseconds and this time istoo short to allow the liquid to fill the cracks. As a result the cracksclose.

Cavitation of a fluid flow pursuant to the proposed method allows not touse setting a packer over the perforation zone (as shown in the FIG. 13)and generate radially (rather than tangentially) directed shock waves atmaximally possible repetition rate. It is high (about 4 kHz) repetitionrate of ‘pumping’ of the cracks what prevents their closing and enablesfurther development (spreading and branching) of cracks long apart fromthe well. Due to this effect drainage of the bottom hole zone isprovided thus increasing the well inflow or injection capacity by up toseveral times.

Another very important advantage of the claimed method is that thecavitation of the fluid flow employing the cavitator devices mounted atthe bottom end of an injection tube column leads to generation andinjecting of the cavitation cavities into a well (where they implode)while the positive pulse of pressure (pressurized stack) is retainedwithin the column. So the imploding of the cavities provides the removalfrom the rock of colmatants that were closing the inflow channels.

Also the cavitation of a fluid flow allows to treat an inner surface ofa tubing and bottom hole zone of a well to remove the carbonaceous orparaffinaceous deposits.

Cavitation allows a local or spatially selective treatment of oilbearing rock to improve inflow from it without affecting the waterinflux zoned resulting in reduction of water content in the producedoil.

The cavitational treatment can last long enough to effect of increase ofinflow in production wells due to treatment of injection wells nearby.

Cavitation of a fluid flow at high values of hydrostatic pressureprovides an opportunity to control the process of a rock breaking whendrilling it with a bit. For example, the tests showed that for drillingwith a rolling cutter drill bit the drilling rate was increased by up totwice and the bit life extended by up to 1.5 times.

Also cavitation of a fluid flow enables low temperature boiling of apumped liquid thus providing an opportunity to decompose at someexcessive pressure the high-weight molecular structures and refine theoil stock or its residual to improve the yield of lighter and volatilecomponents. The carried out experiments show that one can extract up to10-15% of lighter fractions from a residual oil.

APPLICABILITY IN INDUSTRY

This invention can be employed in a borehole to effect the oil-bearingrocks to increment the oil inflow or injectivity; to effect the bottomhole of a well during drilling to stimulate the process of breaking thedrilled rock.

In the first embodiment, a casing of a said device was mounted at thebottom end of tubing and then descended with it into a well to aperforation depth. After washing the well off a ball of a correspondentdiameter was dropped into an inner space of the tubing column. Fluid waspumped until the ball is set on a support restricting further axialmotion within the device casing. Then the hydraulic pumps were connectedto a tubing column to provide pumping rate sufficient to operate thedevice. The treatment of the rock was continued during 6 hours at apumping rate of Q=20 liters per second. The estimated pulse repetitionrate was 4800 Hz.

The results of treatment are as follows: oil inflow rate was increasedby 2.6 times.

In the second embodiment used for rotor drilling a bit sub was mountedabove the bit and inside of which a cone shaped cavitator was placed.The goal of the test was to estimate the effect of the cavitationalregime of washing on effectiveness of breaking the rock by a rollingcutter drill bit. The drilling was carried out at a depth of 1624-1950meters through the monotypic argillite packs at a weight on the bit of18 tons. Bit rotation was 65 rpm, pumping rate was 30-35 liters persecond. A clayey drilling fluid was used of 1.17 g/cub.cm by gravity andviscosity of 30 sec as measured using a SPV-5 cone.

The results of the claimed method implementation in comparativeconditions are as follows: the life of drill bit was extended from 168meters to 319 meters and drilling rate was increased from 1.65 metersper hour to 3.5 meters per hour.

What is claimed is:
 1. Device to cavitate flow of a fluid in a deliveryconduit containing a hydrodynamical cavitator which hydrodynamicalcavitator comprises a working body placed in the channel of the conduitand having an opportunity to move freely in any radial direction of theconduit and a restriction of motion in axial direction of the conduit,and the area of maximal cross section of said working body in a plane,perpendicular to axis of said conduit is more than 0.8 of thecross-section of the conduit but not equal to it.
 2. A method forproducing fluid cavitation in a delivery conduit comprising the stepsof: (a) pumping a fluid through the delivery conduit; (b) placingwithing said conduit a cavatating element comprising a working bodyradially movable inside the conduit and to a limited extent movablealong an axis of the conduit, said working body having a maximumcross-section surface in a plane perpendicular to the axis of theconduit greater than 0.8 of, but not equal to, the cross-section of theconduit; (c) accelerating fluid flow in a respective cross-section of achannel of the delivery conduit to a speed at which Re>Re_(cr), where Reis the Reynolds number and Re_(cr) is the critical Reynolds number; and(d) interrupting the flow for a time less than half-phase of hydraulichammer.
 3. The method of claim 2 further comprising introducing nucleiof cavitation into the fluid selected from the group consisting of gasbubbles, a dispersion of solid particles, and an emulsion of insolubleliquid.